Multi-compartment reactors such as autoclaves are used in numerous chemical processes, and are commonly used in hydrometallurgical processes to recover metal values from aqueous slurries of ores or concentrates. For example, multi-compartment autoclaves are typically pressurized, cylindrical vessels having a plurality of fixed dividers separating adjacent compartments, with each compartment having an agitator and means for injecting an oxidizing gas into the stirred slurry.
As the slurry flows through the autoclave, it passes sequentially through each successive compartment until it reaches the last compartment, from which it is withdrawn as a product mixture for further processing. The slurry must have a sufficient residence or retention time in each compartment, and a sufficient overall retention time in the autoclave, to ensure that the chemical conversion is as complete as possible, in order to maximize metal recovery. The retention time in each individual compartment and the overall retention time in the autoclave are proportional to volume and inversely proportional to flow rate.
Autoclaves have traditionally been configured to permit cascading flow of slurry over the tops of the dividers. Autoclaves of this type are sometimes referred to herein as “overflow autoclaves”. The dividers in an overflow autoclave are progressively decreased in height throughout the length of the autoclave to provide a head drop between adjacent compartments. The slurry level and volume of each compartment except the last compartment are fixed by the heights of the dividers. The flow rate is largely determined by the height of the head drop between the compartments, with some limited variability in flow rate being caused by fluctuations in the feed rate of slurry entering the autoclave. Therefore, the flow rate is substantially constant or fixed. While the liquid level in the last compartment can be controlled by varying the rate at which slurry is withdrawn from the autoclave, this is largely done to compensate for fluctuations in the slurry feed rate. Therefore, the retention time in an overflow autoclave is largely determined by fixed parameters of the autoclave, and cannot be varied or controlled in any significant way.
Autoclaves are also known in which the dividers are provided with openings below the level of the slurry (referred to herein as “underflow openings”), in order to permit at least some of the slurry to flow through, rather than over, the dividers. For example, some overflow autoclaves are provided with relatively small openings in the lower portions of the dividers in order to permit movement of coarse particles through the autoclave and avoid buildup of solids within the compartments. For example, the provision of underflow openings is common practice in nickel laterite processing to avoid the buildup and growth of coarse alunite particles in the autoclave compartments, thereby ensuring that the liquid and solid components of the slurry have a similar retention time distribution (RTD). However, in many such autoclaves the majority of the slurry flows over the tops of the dividers, and they are subject to the same limitations in control of retention time as the overflow autoclaves discussed above.
It is also known to provide autoclaves in which most or all of the slurry flows through the underflow openings in the dividers. In this type of autoclave (referred to herein as an “underflow autoclave”), most or all of the dividers extend above the level of the slurry in the compartments. One example of an underflow autoclave in which all the slurry flows through underflow openings in the dividers is disclosed by Adams et al., in a paper entitled “Mixing Optimization of High Pressure Oxidation of Gold Ore Slurries”, presented at the 1998 Randol Gold & Silver Forum. Another example of an underflow autoclave is disclosed in Ji et al., US 2007/0217285 A1, published on Sep. 20, 2007. In Ji et al., all of the dividers may be configured to permit only through-flow of slurry, or the last divider may be configured for overflow of slurry to compensate for fluctuations in feed rate, as in the overflow autoclaves discussed above. Although Ji et al. discuss the beneficial impact of underflow dividers on RTD, both Adams et al. and Ji et al are silent with regard to control of retention time in an underflow autoclave.
Despite the fact that control of retention time in a multi-compartment autoclave can provide significant benefits in terms of process optimization, the prior art is silent as to how retention time control can be achieved in an autoclave, and certain aspects of autoclave design are incompatible with retention time control.